The present invention relates to fabrication of laminar layered structures in the form of ordered arrays or lattices of particles, preferably with lattice spacing on the scale of 10-100 nanometers (nm), known as the sub-micron length scale.
More particularly, the particles are characterized by substantially uniform diameters not exceeding 50 nanometers. Preferably, the particle diameters are in the 8-20 nm size range, and the distribution of particle diameters is  less than 20% (the standard deviation of the distribution of particle diameters). Such particles can, for example, be produced in accordance with the methods described in the aforesaid application by C. Murray and S. Sun.
To the present date, inexpensive and manufacturable patterning of magnetic media in the submicron size scale has been difficult to attain. The limitations of conventional lithographic patterning for dimensions below 0.1 micron (100 nm.) are well known, and are described in xe2x80x9cLithography for ULSIxe2x80x9d, by S. Okazaki, in a review paper (p. 18, vol. 2440, Proceedings of SPIE). Optical lithography with a light source in the deep ultra-violet (xe2x80x9cDUVxe2x80x9d) is expected to serve in circuit and media fabrication for feature sizes no smaller than about 0.05 micron (50 nm). At present, there are no inexpensive methods for lateral patterning/texturing of solid substrates on a 5 to 50 nm scale.
Also previously, it was difficult or impossible to assemble ordered laminar structures or periodic arrays of particles or objects that are very small (5-20 nm, or 0.005-0.02 micron), and to reliably form such a laminar structure by a method that is simple and inexpensive. In addition, methods to adjust or tailor the lattice spacing in the size range 5-100 nm (0.005-0.1 micron) generally do not exist. There are numerous useful applications of such laminar structures. These include high density magnetic recording media, phased array radiation emitters, radiation sensor arrays, and patterns of electrical contacts/connections for high density interconnections between components. Such patterned electrical contacts are useful in the assembly of stacked integrated circuits.
An ideal method to make such laminar structures described above would have the following features:
1. The method is based on well known procedures and applies to patterning over useful areas (1 to 1,000 cm2).
2. The method allows the spacing between nm scale particles or groupings of such particles to be easily adjusted.
3. The method scales up readily from the laboratory to a manufacturable process.
4. The size distribution, as measured by standard deviation, of the nm scale particles may reach about 20%, rather than the narrower size distribution (e.g. 10%) required by other patterning methods.
It is therefore an object of the present invention to provide such laminar structures as well as methods of fabrication which incorporate all of these features.
Accordingly, this invention proposes the use of a lattice layer, which is made of synthetic deoxyribonucleic acid (DNA), and is designed and fabricated by standard synthetic techniques, and forms by self-assembly of appropriately designed DNA segments. Self-assembly of the DNA lattice is performed in water solution, optionally at an air-liquid interface. The assembled lattice is then transferred onto a substrate surface where it is stabilized. This lattice provides lattice cells or sites which can hold one or more of nm-scale particles, which are thereby assembled into an ordered laminar structure that may have many useful applications.
Also disclosed herein is a chemical affinity/blocking method of assembly. In this method, the substrate surface and the particles are both coated with selected molecules which attract the particles to the substrate surface, and which also enable formation of covalent chemical links between the particles and the surface. The DNA lattice acts to xe2x80x9cblockxe2x80x9d the attractive force between particles and the surface, leaving available lattice cells as attractive sites for particle binding and covalent linking only at the open regions in the DNA lattice layer.
A preferred embodiment provides an organized magnetic recording or storage medium with each bit consisting of about four suitable magnetic particles, and with a well controlled spacing between bits of 25 nm. Such a magnetic storage medium may have an areal information density of about 1012 bit/in2 (1 terabit or Tbit/in2.). Each bit occupies about 625 nm2, and consists of about 4 magnetic particles (optionally crystalline) having a diameter of 8-10 nm. The particles may be ferromagnetic particles comprising a metal such as cobalt, iron, manganese, or nickel. A preferred composition is to alloy one or more of these metals with platinum, palladium or samarium. Alternatively, the particles are made of a ferromagnetic oxide, two examples being BaFe12O19 and SrFe12O19. Optionally, each magnetic particle bit is covered with a thin layer of a noble metal (silver, gold, platinum or palladium). A fabrication method for such a magnetic storage medium is also disclosed.
Moreover, a second embodiment having 9 magnetic particles/bit provides an information density of about 5xc3x971011 bit in2 (0.5 Tbit in2), and an area/bit of about 1,400 nm2. Other embodiments with about 16 and 25 particles/bit are also described.
An alternative embodiment of the present invention permits an array of electrical connections between two different parts with each connection made by a metal particle (gold, for example) of diameter 10-50 nm with spacing between said metal particle connections on the scale of 10-50 nm. The metal particle connections are typically arranged in a square lattice pattern.
It is a purpose of the present invention to easily make laminar layered structures of particles (including but not restricted to crystalline ferromagnetic and semiconductor particles) having a substantially uniform diameter not exceeding 50 nm. It is a further purpose of the present invention to adjust, or tailor, the spacing between the particles in the aforesaid laminar structure within the 10-100 nm (0.01-0.1 micron) size range. Both of these two purposes allow fabrication of magnetic recording media, optically emmitting arrays, and parallel electrical connections with a very high areal density. It is still another object of this invention to stabilize said ordered arrays on a solid substrate, and when necessary to protect said arrays with a thin film overcoating.
The present invention provides a laminar layered structure of nanometer scale particles (the second lattice), with the lattice constant controlled by a coincident lattice layer of deoxyribonucleic acid (DNA). The particles typically have diameters (D) in the 5-20 nanometer (nm) range. The DNA lattice (first lattice) is fabricated using standard automated synthetic methods, and is designed to contain specific nucleotide base sequences, said sequences causing the DNA to form an ordered array of openings, or lattice sites, by self-assembly. Self-assembly of the DNA first lattice is at an air-liquid interface, or in solution.
A preferred embodiment is a magnetic recording or storage medium in which the particles are ferromagnetic particles with diameters in the range of 5-20 nm. and said particles are organized in square information bits with each bit consisting of 4, 9, 16, 25 (etc.) particles, and the lattice of bits is stabilized and protected by a deposited thin film hard coating. Such magnetic storage medium can attain areal information storage densities in the 0.1 to 1 terabit per square inch range.
Moreover, according to another embodiment, the invention can be utilized to create a laminar structure of nm-scale particles in selected patterns or regions of a substrate surface, while leaving the remaining regions free of said particles. The purpose is that selected patterns of the substrate can be made with customized properties by selective placement of the ordered arrays of nm-scale particles. To accomplish selective placement of said particles, an affinity coating (enabling the formation of chemical links between the particles and the substrate) on the substrate is patterned using lithographic methods. This coating is removed in selected regions, and left intact in the remaining regions. The pattern of the affinity coating may have any desired shape, either geometric or an arbitrary shape. Then during assembly (as shown in FIG. 5 and FIG. 7), the nm-scale particles adhere to the substrate only in the selected regions containing the intact affinity coating. The nm-scale particles do not adhere in the regions of the substrate where the affinity coating has been removed.
Accordingly, the present invention broadly provides a laminar layered structure comprising:
a) a substrate having a surface,
b) a lattice layer in the form of a lattice disposed upon said surface of said substrate, said lattice layer comprising DNA segments arranged to form cells of said lattice layer,
c) at least one particle disposed within each cell.
Preferably, particles within cells of the lattice layer have a substantially uniform diameter not exceeding 50 nanometers. For the purpose of particle diameters of the present invention, the term xe2x80x9csubstantially uniformxe2x80x9d shall mean that the standard deviation of particle diameters should not exceed 20% of the mean particle diameter. Such tolerances of particle diameter are desireable to facilitate the the disposition of particles within cells of the lattice
Preferably, the laminar layered structure further comprises
d) an adherent coating disposed over the aforesaid lattice layer and over the aforesaid particles to maintain each particle within a cell of said lattice. Preferably, the adherent coating comprises abrasion-resistant material selected from the group consisting of diamond-like-carbon, amorphous carbon, amorphous silicon, aluminum oxide, and silicon oxide.
Especially for use in a magnetic storage medium, each cell has a diameter not exceeding 100 nanometers and the particles have a substantially uniform diameter not exceeding 20 nanometers. These particles may then comprise a magnetic material selected from the group consisting of elements Co, Fe, Ni, Mn, Sm, Nd, Pr, Pt, Gd, an intermetallic compound of the aforesaid elements, a binary alloy of said elements, a ternary alloy of said elements, an oxide of Fe further comprising at least one of said elements other than Fe, barium ferrite, and strontium ferrite. Moreover, each cell may optionally contain a plurality of said particles, where this plurality is the square of an integer.
According to other embodiments, each particle may comprise a material having a selected degree of electrical conductivity, and the particles may have a substantially uniform diameter not exceeding 50 nm. For example, the material may have a high degree of electrical conductivity for use of the laminar layered structure as an electrical contact layer for contacting a similar layer on a different substrate. Alternatively, the material may be a semiconductor material capable of emitting electromagnetic radiation for use of the laminar layered structure as a phased array emitter. According to another embodiment, the material may be a semiconductor material capable of sensing electromagnetic radiation for use in optical or other radiation detectors.
Moreover, to form a patterned layered structure, an affinity layer is disposed in a selected pattern over at least part of the surface of the substrate, the affinity layer being composed of an affinity material adapted to preferentially attract and retain the particles in the aforesaid selected pattern over the surface. Such an affinity material may preferably comprise bi-functional molecules having a group selected from tri-alkoxysilane and trichlorosilane at one end thereof and, at another end thereof, a group selected from carboxil acid and thiol.
In general, the affinity layer is formed by a layer of molecules which have two active chemical groups which should be selected to bind to the substrate and the nanopaticle surface thereby tethering the particles to the substrate surface. Affinity molecules can be expressed generally in the form X-R-Y where X and Y are the active head groups and R is a hydrocarbon or flourocarbon chain preferably containing 3-22 carbon atoms.
The functional groups X and Y are chosen from:
sulfonic acids Rxe2x80x94SO2OH
sulfinic acids Rxe2x80x94SOOH
phosphinic acids R2POOH
phosphonic acids Rxe2x80x94OPO(OH)2 
carboxylic acids Rxe2x80x94COOH
thiols Rxe2x80x94SH
trismethoxysilane Rxe2x80x94Si(OCH3)3 
trisethoxysilane Rxe2x80x94Si(OCH2CH3)3 
trichlorosilane Rxe2x80x94SiCl3 
In a given affinity molecule the chemical functional groups X and Y may be the same although they are generally not, because the substrate surface and the nanoparticle surface are generally comprised of different materials.
One example of an affinity layer is trismethoxysilylpropane thiol, which may be expressed as (CH3O)3Sixe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94SH, which selectively binds noble metal coated nanocrystals to silicon oxide surfaces.
The present invention further provides a method of forming a laminar layered structure upon a surface of a substrate comprising the steps of:
a) coating said substrate with an affinity coating,
b) preparing a solution (e.g. aqueous or CsCl) of a selected group of four DNA segments each formed from a base molecules of adenine (A), guanine (G), cytosine (C), and thymine (T),
c) bringing said surface into contact with said solution to thereby apply a layer of said solution to said surface,
d) drying said layer to leave a lattice layer of cells formed by DNA segments upon said surface,
e) preparing a liquid dispersion of inorganic particles coated with an organic stabilizer material, said inorganic particles having a substantially uniform diameter not exceeding 50 nanometers.
f) applying said liquid dispersion to said surface of said substrate to cause at least one of said inorganic particles coated with said organic stabilizer material to adhere to said surface within each cell of said lattice layer, said particles being maintained in spaced-apart relationship upon said surface by said organic stabilizer material.
Moreover, it may be preferable to then remove the aforesaid organic stabilizer material, and depositing an adherent coating over said particles to maintain them in the aforesaid substantially uniformly spaced-apart relationship. Such removal of the organic stabilizer material may be carried out by evaporation using at least one of heating, dry etching, and vacuum.
In general, possible organic stabilizers for nanoparticles are long chain organic compounds which may be expressed in the form R-X where:
(1) R-a xe2x80x9ctail groupxe2x80x9d which is either a straight or branched hydrocarbon or flourocarbon chain.
R-typically contains 8-22 carbon atoms
(2) X-a xe2x80x9chead groupxe2x80x9d which is a moiety (X) which provides specific chemical attachment to the nanoparticle surface. Active groups could be sulfinate (xe2x80x94SO2OH), sulfonate (xe2x80x94SOOH), phosphinate (xe2x80x94POOH), phosphonate xe2x80x94OPO(OH)2, carboxylate, and thiol.
Thus the stabilizers which result are: sulfonic acids
Rxe2x80x94SO2OH
sulfinic acids Rxe2x80x94SOOH
phosphinic acids R2POOH
phosphonic acids Rxe2x80x94OPO(OH)2 
carboxylic acids Rxe2x80x94COOH
thiols Rxe2x80x94SH
One specific choice of organic stabilizer material is oleic acid.